Count date-like strings in files in /var/log

log-volume-by-date.sh

#!/bin/bash

set -euo pipefail

log_directory="${1:-/var/log}"

year="20[[:digit:]]{2}"
day="[[:digit:]]{2}"
month_num="[[:digit:]]{1,2}"
month_alpha="[[:upper:]][[:lower:]]{2}"
separator="[-\/]"
# 2020-06-29
# 2020/06/29
iso_8601="${year}${separator}${month_num}${separator}${day}"
# 30/Jun/2020
american="${day}${separator}${month_alpha}${separator}${year}"
date_pattern="(${iso_8601}|${american})"

# * `grep`: search log files in `LOG_DIRECTORY` for things that look like dates
#   (`date_pattern`); print the lines that match
# * `sed`:
#   * remove anything after the first date in the line
#   * remove anything before the first date in the line
#   * replace slashes with dashes so `date` can parse dates that have a month's
#     name instead of a number (`30/Jun/2020` => `30-Jun-2020`)
# * `date`: parse each line of standard input as a date and print it in
#   `YYYY-MM-DD` format
# * `sort`: if lines aren't sorted `uniq` treats separated chunks of duplicate
#   text as separate things
# * `uniq`: count how many times each date appears
# * `sort`: sort by count
grep \
  --color=never \
  --dereference-recursive \
  --extended-regexp \
  --no-filename \
  "${date_pattern}" \
  "${log_directory}" \
  2>/dev/null \
  | sed \
    --quiet \
    --regexp-extended \
    -e "s/(${date_pattern}).*$/\1/p" \
  | sed \
    --quiet \
    --regexp-extended \
    -e "s/^.*(${date_pattern})/\1/p" \
  | sed 's:\/:-:g' \
  | date --file=- +'%F' \
  | sort \
  | uniq --count \
  | sort --key=1 \
    --numeric \
    --reverse